Known systems for spin-lock magnetic field preparation that generate T1rho contrast associated with a spin lattice relaxation time constant in a rotating frame, are limited by SAR (Specific Absorption Rate) constraints. This is because a long radiofrequency (RF) pulse is needed to generate the required T1rho contrast. The known systems are also vulnerable to inhomogeneity in the static magnet field (B0) and also a dynamic transmission RF field (B1). Attempts to reduce resulting artifacts involve longer imaging time and increased risk of patient motion and mis-registration artifacts. A system according to invention principles addresses these deficiencies and associated problems and provides improves spin-lock magnetic field preparation.